The purpose of this project is to evaluate the effectiveness of a structured aerobic exercise intervention for adults with Attention-Deficit/Hyperactivity Disorder (ADHD) with and without medication and compare it to medication alone. Participants will be randomly assigned to medication only + education, aerobic exercise intervention only, and combined aerobic exercise and medication groups. Participants will be evaluated at baseline, following medication optimization (for medicated groups), following 8 weeks of intervention, after 3 months of follow-up, and after 6 months of follow-up. The investigators hypothesize that the combined group will have the best outcome at all evaluation points and that treatment gains will be maintained throughout the follow-up period if the assigned treatments are continued.

Background: Several interventional studies have shown improvement in core symptoms of Attention-Deficit Hyperactivity Disorder (ADHD) with aerobic exercise as an add-on therapy for children with ADHD treated with stimulants. Studies in the general population have shown that aerobic exercise in adults may improve cognitive function in general and executive function in particular. Although non-medication treatment for ADHD has focused on cognitive training in addition to medication, some evidence indicate that intense aerobic exercise impacts brain structure, improves brain function, and has effects similar to stimulant medication.

Objectives: To assess the effectiveness, regarding ADHD symptoms, of an eight week aerobic exercise intervention, medication, and the combination of both aerobic exercise and medication treatments for adults with ADHD. The secondary objectives are to determine the effectiveness of these treatments on social skills, as measured by functioning in work, social, and family situations; anxiety symptoms; depressive symptoms; self-esteem; and the effect of motivation for exercise on adherence to exercise treatment.

Hypothesis: Investigators hypothesize that the combined group which receives aerobic exercise and medication intervention will have the greatest improvement in both primary and secondary outcome measures; that treatment gains will be maintained at follow-up only if treatments are continued.

Methods: Participants will be randomly assigned to one of three groups:

1. Aerobic exercise intervention only. Participants of this group will meet twice a week for 8 consecutive weeks. Each meeting will involve 1 hour of moderate-vigorous intensity aerobic exercise.
2. Medication treatment only. Participants of this group will receive optimal medication for them and will gradually be titrated up to their optimal dose. Once they are optimally titrated, they will attend 8 weekly sessions of an education class, which will focus on different topics of adult ADHD. These meetings will last 1 hour each week. Educational meetings are meant to control for group effects of the exercise group intervention.
3. Medication combined with aerobic-exercise intervention. Participants of this group will receive optimal medication for them and will gradually be titrated up to their optimal dose. When the participants of this group will be at their optimal medication dose, they will begin the aerobic exercise classes twice a week for 8 weeks. They will not receive the educational sessions.

Randomization will be stratified by sex, age and the degree of physical activity per week. Power analyses indicate that 23 participants are needed per each treatment arm, accounting for a dropout rate of 20%. Therefore initial enrollments should be at minimum 69 patients for all three arms, at least 23 patients per arm. There will 2 groups in each arm of the study, accounting for 10-15 participants per group at any one time.

Inclusion Criteria:

1. Diagnostic and Statistical Manual of Mental Disorders - Fifth Edition (DSM-5) criteria for adult ADHD of any of three presentations (inattentive, hyperactive impulsive, combined) via Conners Adult ADHD Diagnostic Interview for the DSM-5 (CAARD-D) and clinician's assessment;
2. Barkley Childhood and Current Symptom of ADHD (1998) completed by self and informants (parents or siblings for childhood symptoms, spouses or co-workers for current symptoms). Required cut off on these scales are scores 1.5 Standard Deviation above relevant gender and age norms;
3. Conners Adult ADHD Rating Scale (1999) completed by self and informants, such as spouses or co-workers for current ADHD symptoms. Required cut off on these scales are scores 1.5 Standard Deviation above relevant gender and age norms;
4. Between 18 and 60 years old
5. Be able to give informed consent and comply with study procedures;
6. I.Q. of 85 or above on Wechsler Adult Intelligence Scale - Third Edition (WAIS-III) by three verbal and three nonverbal subtests
7. Adequate command of English to be able to participate in the group treatment.
8. Written permission from a family physician to participate in aerobic exercise and no contra-indications for stimulant medication.

Exclusion Criteria:

1. Psychotic symptoms, past or current;
2. Current psychiatric comorbidity, which needs ongoing medication treatment e.g. bipolar disorder, major depression, suicidality, anxiety, current substance use disorder (must be free of substance abuse for 12 months);
3. Medical condition that preclude use of the stimulant medication, e.g. hypertension, cardiac disease, Tourette syndrome, etc.
4. Organic mental disorders or other significant neurological disorders, e.g. epilepsy, head injury, chorea, multiple sclerosis.
5. Pregnant or breastfeeding women.
6. Patients currently involved in intensive aerobic exercise, two hours per week or more.
7. Patients who are currently effectively treated for ADHD.